Cosmological Constant

Definition: The cosmological constant is a constant term in field equations of general relativity, represented by the Greek symbol Lambda, which allowed for a static universe. Later evidence supported the fact that the universe was indeed expanding and the cosmological constant was believed to be zero. Evidence in the late 1990s has begun supporting the idea that the universe is not only expanding, but that the expansion rate is actually accelerating due to the presence of dark energy.

Einstein's Biggest Blunder

When Albert Einstein developed his theory of general relativity, he realized that they implied an unstable equilibrium position. Any slight unevenness would cause spacetime to expand or contract. He had the philosophical belief (as did most physicists of the time) in a static universe, so he added a constant term which was allowed (but not required) onto the end of his equation when he published the theory in 1916.

In 1929, however, the astronomer Edwin Hubble discovered evidence that distant galaxies were receding from our own galaxy. Though Einstein's model, with the cosmological constant, other models by Alexander Friedmann and Willem de Sitter (which didn't include the cosmological constant) had predicted such expansion quite clearly. Einstein quickly accepted the new evidence and told physicist George Gamow that the cosmological constant idea was the "biggest blunder" of his life.

Cosmological Constant Revisited

In 1998, two different teams of researchers discovered evidence that the universe's expansion was actually speeding up. This meant that the cosmological constant wasn't just zero, as expected, but had to have a very slight positive value. The theory that has grown up around this positive cosmological constant is the theory of dark energy.